1-hexene and 1-octene, which are monomers or comonomers for producing linear low-density polyethylene, are an important industrial material widely used in a polymerization process, and are obtained by purifying products resulting from oligomerizing ethylene. However, conventional ethylene oligomerization is inefficient in terms of producing considerable amounts of butene, higher oligomers, and polyethylene, as well as 1-hexene and 1-octene. Such conventional ethylene oligomerization techniques typically produce a variety of α-olefins depending on the Schulze-Flory or Poisson product distribution, undesirably obtaining desired products in limited yield.
Recent research into the production of 1-hexene or 1-octene by selectivity trimerizing or tetramerizing ethylene using transition metal catalysis is ongoing, in which almost the known transition metal catalysts are chromium-based catalysts. International Patent Publication WO 02/04119 discloses an ethylene trimerization catalyst, namely, a chromium-based catalyst using a ligand of (R1)(R2)X—Y—X(R3)(R4), in which X is phosphorus, arsenic or antimony, Y is a linker such as —N(R5)—, and at least one of R1, R2, R3 and R4 has a polar substituent or an electron donating substituent.
Another publication discloses the use of a (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2 compound in which at least one of R1, R2, R3 and R4 has no polar substituent, as a ligand which does not exhibit catalytic activity for 1-hexene under catalytic conditions (Antea Carter et al., Chem. Commun., 2002, p. 858-859).
In addition, Korean Unexamined Patent Publication No. 2006-0002741 discloses that superior activity and selectivity of ethylene trimerization are actually possible using a PNP ligand containing a non-polar substituent at an ortho-position of a phenyl ring attached to phosphorus, such as (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2.
Also, International Patent Publication WO 04/056479 discloses increasing selectivity in the production of 1-octene by tetramerizing ethylene using a chromium-based catalyst containing a PNP ligand having no substitutent on a phenyl ring attached to phosphorus. As such, a heteroatomic ligand used for an ethylene tetramerization catalyst is exemplified by (phenyl)2PN(isopropyl)P(phenyl)2 or the like
This conventional publication discloses that the chromium-based catalyst containing a heteroatomic ligand having nitrogen and phosphorus heteroatoms enables ethylene to be tetramerized even in the absence of a polar substituent on the hydrocarbyl or heterohydrocarbyl group bonded to the phosphorus atom, thus producing 1-octene at a selectivity exceeding 70 mass %.
However, the conventional techniques do not propose definite examples of the heteroatom-containing ligand structure which is capable of highly selectively tetramerizing or trimerizing ethylene to produce 1-octene or 1-hexene. As well, these techniques propose only a PNP-type backbone structure, such as (R1) (R2)P—(R5)N—P(R3)(R4) as a ligand having 1-octene selectivity of about 70 mass %. Furthermore, the forms of substituents that are substitutable in the heteroatomic ligands are limitedly provided. Specifically, although the tetramerization selectivity greatly depends on a bridge structure between a P atom and another P atom in the ligand backbone structure, the conventional techniques disclose that the catalyst is defined as a highly selective catalyst so long as P atoms are respectively connected to both sides of the bridge structure.
Also, the conventional PNP type backbone ligands having heteroatoms are problematic because in the preparation of 1-octene or 1-hexene the reaction activity is not uniformly maintained over time and the reaction rate is remarkably decreased. This is considered to be because the N atom of the ligand backbone structure may be easily coordinated with the transition metal due to the presence of an unshared electron pair thereof and thus may be adapted for a ligand but may be induced to easily dissociate from the transition metal by means of the P atom having comparatively poor coordination performance. The publication discloses facilitating the PNP backbone ligand to be converted from P—N—P into N═P—P depending on the synthesis conditions including the solvent and substituent polarity (Dalton Trans., 2003, 2772).
Also, another publication discloses that in the case of a heteroatom-containing PNP type backbone ligand, when ethylene oligomerization is performed using a catalyst complex already synthesized from the ligand and a chromium precursor, the reaction activity and selectivity are not greatly changed compared to when the ligand and the chromium precursor are separately added (J. Am. Chem. Soc., 2004, 126, 14712).
However, the aforementioned publications disclose only the very limited catalyst to actually prepare 1-hexene and 1-octene at high activity and high selectivity, in which the activity thereof is low, resulting in confined industrial applicability. In particular, the known catalysts are problematic because the expensive co-catalyst exemplified by methylaluminoxane is used, making it more difficult to achieve industrialization.